There are many situations where it is desirable to provide audio output for personal use to be worn or carried near the body. This audio output could be used for portable entertainment, personal communications, information and the like. These personal and portable communication and entertainment products include, for example, cellular and portable radios, tape players, CD players and audio portions of portable video systems and personal monitors.
The audio output for many of these systems is typically directed to the wearer through the use of transducers physically positioned in the ear or covering the ear, such as earphones and headphones. Earphones and headphones, however, are often uncomfortable to use for long periods of time. Also, they can block or attenuate environmental sounds causing the wearer to lose contact with the surroundings. This can compromise safety considerations if the wearer is engaging in activities such as jogging, driving a vehicle, or operating machinery. Further, the feeling of being partially disconnected from one's audio environment often adds to the discomfort of conventional headphones and earphones.
One common use of audio systems with earphones and headphones involves exercise and athletic events. It is quite common to see people jogging or exercising with headphones or earphones positioned in or covering their ears. Not only is this dangerous since the person often loses contact with external sounds and surroundings, but the earphones and headphones are subject to being dislodged as a result of the activity.
It is commonly desired to provide stereo audio output from these portable entertainment and personal communication systems. Also, a stereo audio output may be provided without earphones or headphones by arranging small loud speakers (a/k/a transducers) on the body. These small speakers, however, are not able to create broad-band high fidelity sound, particularly in the low frequency ranges, and furthermore will radiate a substantial amount of sound into the wearer's surroundings, which can be unacceptably annoying to others in the vicinity. In this regard, loud speaker transducers are usually mounted in enclosures to confine the acoustic radiation from the rear portions of the transducer so that the radiation does not combine with out-of-phase radiation from the front portions of the transducer. Without such an enclosure, there is a significant reduction of net radiated intensity, especially in the low frequency audio ranges.
For wearable speakers, the requirement of an enclosure creates a problem. In general, the volume of the enclosure will be quite small and its acoustic stiffness will dominate the speaker behavior. The result will be a high resonance frequency and consequently a poor low frequency response.